1. Field of the Invention:
The present invention relates to an S-alkyl thiocarbamate base resin useful for making lenses, and a process for producing an S-alkyl thiocarbamate base resin. The invention also relates to a plastic lens comprising the resin, and a process for making the lens.
Plastic lenses are lightweight, less fragile and easier to dye than inorganic lenses, and thus have been widely used in recent years as optical elements such as eyeglass lenses and camera lenses.
2. Description of the Prior Art
A resin currently widely used for producing optical elements is formed by subjecting diethylene glycol bis(allylocarbonate) (hereinafter referred to as DAC) to radical polymerization. This resin has excellent impact resistance, is lightweight, easy to dye and exhibits superb processability, and machinability including cutting and polishing ability.
However, the resin has a lower refractive index (n.sub.D =1.50) than inorganic lenses (n.sub.D =1.52). In order to obtain equivalent optical properties to glass lenses, the center thickness, peripheral thickness and curvature of a lens made of the resin must be increased and thus the lens becomes very thick. Accordingly, there has been an urgent need to develop resins useful for making lenses that have higher refractive indices.
Resins having higher refractive indices that have been used to produce plastic lenses include a polyurethane resin obtained by reacting isocyanate compounds with hydroxyl compounds such as diethylene glycol (Japanese Patent Laid-Open No. 136601/1982 (USP 4443588)), with halogen containing hydroxyl compounds such as tetrabromobisphenol-A (Japanese Patent Laid-Open No. 164615/1983), with sulfur containing hydroxyl compounds (Japanese Patent Laid-Open Nos. 194401/1985 and 217229/ 1985 (USP 4680369, USP 4780522)), or with polythiol compounds (Japanese Patent Laid-Open Nos. 199016/1985 (USP 4689387) and 267316/1987).
Although lenses made of these prior art resins have improved refractive indices over lenses made of DAC, their refractive indices are still insufficient. Moreover, these lenses exhibit high dispersion of refrective index, poor weatherability, or large specific gravity, due to the presence of several halogen atoms or aromatic rings for the purpose of improving the refractive index of the resulting lens.
Casting polymerization is generally used to produce plastic lenses prepared from DAC, PMMA, and polycarbonates. In the preparation of such lenses, it is known to add butyl phosphate to DAC as an internal mold releasing agent to improve the mold releasing property. In general, however, internal molding agents are not used since they often result in diminished quality of appearance of such molded articles [Seiichi Mima, Polymer Digest, 3, 39 (1984); etc.].
On the other hand, high refractive indices can be expected with polyurethane lenses made of the resin having S-alkyl thiocarbamate bonds. However, the resin adheres to a mold intimately upon molding, so that it is generally difficult to release it from the mold after polymerization. With this phenomenon in view, the present inventors have previously proposed a process in which an external releasing agent is used (Japanese Patent Laid-Open No. 267316/1987, etc.) or a process in which a polyolefin resin mold is used (Japanese Patent Laid-Open No. 236818/1987) to improve mold releasing properties in the preparation of lenses made of an S-alkyl thiocarbamate base resin formed by reacting a polyisocyanate and a mercapto compound.
However, these processes are still insufficient for improving the mold releasing property in casting polymerization of the S-alkyl thiocarbamate base lens of the present invention.
Specifically, in prior art processes in which external molding agents are used, a portion of the agent that is applied to the inner surface of a mold migrates to the surface or interior of the polymerized lens which results in irregularity or unevenness on the surface of the lens or turbidity within the lens. When the mold is used repeatedly, mold releasing treatment is required in each molding cycle, thus making the process complicated and decreasing productivity in producing the lens, rendering the process industrially unpractical. When the polyolefin resin mold is applied, the resin mold deforms at elevated temperatures which causes profile irregularities on the surface of the molded lens.
Thus, the lens is not useful in applications which require a high accuracy on the molded surface.